1. Field of the Invention
The present invention relates to an imaging lens that captures the image of an object, and an imaging apparatus using the imaging lens.
2. Description of the Related Art
An in-vehicle imaging apparatus has been proposed which monitors the front of a vehicle. The imaging apparatus is provided in the vehicle to monitor the deviation of the vehicle from its lane or to monitor traffic signs while the vehicle is traveling. Generally, an imaging lens having a small size and a long back focal length has been used as an in-vehicle imaging lens.
For example, a structure has been proposed in which a cemented lens or a plastic aspheric lens is used as an imaging lens having a small size and a long back focal length (see Japanese Patent Nos. 3254239, 3478643(U.S. Pat. No. 5,682,269), 3723637, and 3964533).
Japanese Patent Nos. 3254239, 3478643(U.S. Pat. No. 5,682,269), 3723637, and 3964533 disclose imaging lenses using cemented lenses. Japanese Patent No. 3254239 discloses an imaging lens that has a large number of lenses including an aspheric lens having a relatively small angle of view. Japanese Patent No. 3478643(U.S. Pat. No. 5,682,269) discloses an imaging lens using a large number of meniscus lenses. Japanese Patent No. 3723637 discloses an imaging lens having a total length greater than the focal length. Japanese Patent No. 3964533 discloses an imaging lens that has a relatively high F number (Fno=4) and a small angle of view and does not correspond to an increase in diameter.
However, the in-vehicle imaging lens, for example, is provided in the vehicle in severe environments, such as a low-temperature environment of 0° C. or less in a cold region and a high-temperature and high-humidity environment in a tropical region (for example, a temperature of 80° C. and a humidity of 80%). Therefore, a cement of the cemented lens or the plastic lens is made of a material forming the imaging lens is likely to be transformed or deformed, which may cause deterioration of the optical performance of the imaging lens, for example, a reduction in resolution. In addition, the optical performance of the imaging lens may be lowered due to a variation in the shape or refractive index of the imaging lens caused by a change in the temperature of the imaging lens while capturing the image of an object.
Therefore, it is necessary to prevent the optical performance of the imaging lens from being lowered due to the transformation or deformation of each lens, or the variation in the refractive index of each lens when the imaging lens is provided in a severe environment.
The problem of the deterioration of the optical performance of the imaging lens is not limited to the in-vehicle imaging lens, but it also arises in an imaging lens including a plastic lens or a cemented lens.